Sulfur tetrafluoride is a chemical compound with the formula SF4. It is a colorless corrosive gas that releases dangerous hydrogen fluoride gas upon exposure to water or moisture. Sulfur tetrafluride is a useful reagent for the preparation of organofluorine compounds,[3] some of which are important in the pharmaceutical and specialty chemical industries.
Sulfur in SF4 is in the +4 oxidation state, with one lone pair of electrons. The atoms in SF4 are arranged in a see-saw shape, with the sulfur atom at the center. One of the three equatorial positions is occupied by a nonbonding lone pair of electrons. Consequently, the molecule has two distinct types of F ligands, two axial and two equatorial. The relevant bond distances are S–Fax = 164.3 pm and S–Feq = 154.2 pm. It is typical for the axial ligands in hypervalent molecules to be bonded less strongly.
The 19FNMR spectrum of SF4 reveals only one signal, which indicates that the axial and equatorial F atom positions rapidly interconvert via pseudorotation.[4]
At the laboratory scale, sulfur tetrafluoride is prepared from elemental sulfur and cobaltic fluoride[5]
S + 4CoF3 → SF4 + 4CoF2
SF4 is industrially produced by the reaction of SCl2 and NaF with acetonitrile as a catalyst[6]
3 SCl2 + 4 NaF → SF4 + S2Cl2 + 4 NaCl
At higher temperatures (e.g. 225–450 °C), the solvent is superfluous. Moreover, sulfur dichloride may be replaced by elemental sulfur (S) and chlorine (Cl2).[7][8]
A low-temperature (e.g. 20–86 °C) alternative to the chlorinative process above uses liquid bromine (Br2) as oxidant and solvent:[9]
Inorganic synthesis, SF4 is used to convert COH and C=O groups into CF and CF2 groups, respectively.[10] The efficiency of these conversions are highly variable.
In the laboratory, the use of SF4 has been superseded by the safer and more easily handled diethylaminosulfur trifluoride, (C2H5)2NSF3, "DAST":[11] This reagent is prepared from SF4:[12]
^Holleman, A. F.; Wiberg, E. (2001). Inorganic Chemistry. San Diego: Academic Press. ISBN0-12-352651-5.
^Kwasnik, W. (1963). "Fluorine compounds: Sulfur tetrafluoride". In Brauer, Georg (ed.). Handbook of Preparative Inorganic Chemistry. Vol. 1. Translated by Riley, Reed F. (2nd ed.). NY, NY: Academic Press. p. 168. LCCN63-14307 – via the Internet Archive.
^Tullock, C. W.; Fawcett, F. S.; Smith, W. C.; Coffman, D. D. (1960). "The Chemistry of Sulfur Tetrafluoride. I. The Synthesis of Sulfur Tetrafluoride". J. Am. Chem. Soc. 82 (3): 539–542. doi:10.1021/ja01488a011.
^US 2992073, Tullock, C.W., "Synthesis of Sulfur Tetrafluoride", issued 1961
^Winter, R.W.; Cook P.W. (2010). "A simplified and efficient bromine-facilitated SF4-preparation method". J. Fluorine Chem. 131: 780-783. doi:10.1016/j.jfluchem.2010.03.016
^Fauq, A. H. (2004). "N,N-Diethylaminosulfur Trifluoride". In Paquette, L. (ed.). Encyclopedia of Reagents for Organic Synthesis. New York: J. Wiley & Sons. doi:10.1002/047084289X. hdl:10261/236866. ISBN9780471936237..
